The idea of providing lids for ladles is not new. Reference may be had to an article entitled "Operating Results Obtained with Dolomite and Bauxite Ladle Linings in the BOS Steelmaking Plant of the Dillinger Huttenwerke AG", by K. H. Bauer and R. Quinten, published in Stahl Eisen 1980 100 (18), pp. 1045-1050.
Generally speaking, the advantages of providing a lid for a molten metal ladle can be summed up under the following headings: better heat retention, improved life of higher quality refractories, and increased crane availability. In installations where no lids are provided for ladles, an individual cleaning out a ladle stands behind a large shield which is fixed in a permanent location, while an overhead crane supports the ladle in front of the shield. In some installations, when one crane is immobilized, other cranes on the same crane rails can also be restricted. In essence, a ladle lid eliminates the necessity for a shield, thus resulting in increased crane availability, since the crane is no longer engaged in holding the ladle during cleaning.
In addition to the self-evident improved heat retention, there tends to be less "pancaking" or "skulling" in the ladle. A "pancake" is a flat and usually circular mass of solidified steel or slag that "freezes" in the ladle, usually on the bottom, as a result of temperature being insufficient to prevent solidification. "Pancaking" is the formation of "pancakes".
"Skulling" similarly refers to the formation of "skulls". These are basically pancakes with edges. They can also refer to the shell of metal that occasionally solidifies on the sides and bottom of the interior of the ladle.
Furthermore, the provision of a ladle lid permits the use of higher quality refractories in the ladle that are more susceptible to spalling. Spalling results from inadequate resistance to thermal shock, and is the breaking away of pieces of refractory from the hot face, thus exposing fresh surfaces. Spalling may result from a too-rapid expansion or contraction of the hot face of the refractory with sudden temperature changes. This type of spalling is called thermal spalling. By providing a ladle lid, heat retention in the ladle refractory is improved, thus minimizing thermal shock.
It will be understood that, generally, there is always a certain amount of residual heat in a ladle between heats even when the ladle is empty. If the empty ladle remains uncovered, approximately 70% of the heat is lost to the surrounding atmosphere by radiation, with the remaining 30% of the heat loss occurring as a result of conduction from the hot face of the refractory through to the inner colder refractory adjacent the steel ladle shell. If the same empty ladle were covered, the radiation losses would be eliminated, and the loss resulting from conduction into the colder refractory would be reduced.
The above heat losses are different from the losses occurring when the ladle contains molten steel. When the ladle contains molten steel and is uncovered, approximately 30% of the heat loss results from radiation losses from the molten steel or slag surface to the atmosphere, whereas approximately 70% of the heat is lost by conduction to the refractory from the surfaces in contact with the refractory walls and bottom. When an empty hot ladle is filled with molten steel and covered, radiation losses are again eliminated, and the conduction temperature losses to the refractories are significantly reduced, because of the high refractory temperature.
As is well known, the typical furnace installation for making steel includes a relatively large vessel from which steel is periodically tapped into ladles. A typical ladle may be approximately 18 feet high, and from 10 to 12 feet in diameter at the top. Normally, a ladle is supported and transported on a ladle transfer car which runs on tracks along a path which takes it under the steelmaking vessel. The ladle serves as an intermediate container to transfer steel from the vessel to a teeming location. The teeming may take place into ingot molds or into a tundish for continuous casting.
The temperature of the steel when first tapped from the vessel must be high enough to ensure that it will remain above a predetermined casting temperature for the time during which steel remains in the ladle. Naturally, if the ladle has an open top, considerable heat is lost from the melt, and the temperature of the melt will drop more rapidly over time. The losses have already been discussed above in detail.
In determining an appropriate construction for such a ladle lid, however, certain factors must be kept in mind. Firstly, it is obviously preferable to be able to retrofit lids onto existing ladles, certainly from an economic point of view. Most ladles are constructed in such a way that the large trunnion pins, located at diametrically opposed positions on the ladle, are close to but slightly above the center of gravity of the ladle when empty. This means that the procedure of dumping the ladle does not require excessive tipping force to be applied by the crane when raising the bottom end. The fact that the trunnion pins are close to the center of gravity, however, means that any retrofitted lid must not be excessively heavy, for fear that it would cause an over-balancing or over-tipping of the ladle when a certain angle is reached during the tipping procedure. In other words, the ladle lid must not be so heavy that the center of gravity is raised to a location above the axis of the trunnion pins.
Another factor to be kept in mind relates to the necessity of lifting the ladle lid off the ladle prior to running the ladle under the steel making vessel for tapping, and then replacing the lid on the ladle after the ladle comes out from under the vessel. Naturally, this could be done using a crane hook, but such a procedure would tie up a crane which might otherwise be made better use of elsewhere in the plant. Thus it would be desirable to provide a special lifting apparatus which will automatically remove the lid from the ladle as it approaches the steelmaking vessel, and which will allow the lid to be placed back on the ladle, again automatically, as the ladle comes out from under the vessel. By "automatically" is meant that the lid is removed from and replaced on the ladle by the power of the ladle transfer car.